encoding

I'm about to make a new font and I'm not sure which encoding to use.
I will have Latin characters + some custom ligatures + alternate versions + small capitals.

Should I use ISO-8859-1 with additional characters or Unicode ?

If using Unicode, all the codes seem to be dedicated to some usage. And i'm not sure where to place my custom characters. Are there some rules for that ?

For example, I put my first custom ligature at U+FB07, because the unicode code charts says Latin Ligatures start at U+FB00 with only 7 foreseen. Is that the way to do or am I just making to much fuss about this and should I just add some extra places at the end of the font for my own needs ?